Programmable analog hearing aids have been in use for a number of years. These hearing aids allow precise adjustment of the specific parameters of a hearing aid processing scheme to achieve a reasonably good "fit" for the hearing aid user. Programmable digital hearing aids extend this capability by also allowing new programs to be downloaded. The ability to load a new program on a digital hearing aid means that entirely different processing schemes can be implemented simply by downloading new software.
Hearing aids have traditionally been programmed with wired links that sometimes connect to a body worn programming interface that in turn incorporates a wired or wireless link to the hearing aid programmer. The use of a wired link means that a hearing aid must incorporate a connector for the programming cable. Typical programming interfaces use serial data transmission with between two and four electrical connections depending on whether the serial connection is transmit and receive or receive-only. Newer connection schemes that do not require a separate programming connector have recently been developed. They use the battery terminals to supply power and transmit data to the hearing aid. This approach sometimes requires that additional battery contacts be added, depending on the nature of the serial interface. All of these programming methods require special programming cables and small connectors that are expensive and prone to breakage.
Other programming interfaces that have been used successfully are infrared or ultrasonic links. All of these approaches require additional circuitry increasing costs and power consumption and the space occupied within the hearing aid. For digital hearing aid programming, ultrasonic links are not practical because of the high sampling rate required to convert and ultrasonic signal into a digital representation. Although they are often used to transmit data between programming interfaces and personal computers, infrared links have never been widely used on hearing aids because of their higher power consumption, susceptibility to interference and undesirable directional characteristics. Thus, the majority of current digital hearing aids rely on wired programming links which require a specialized connector and programming cable.
An important consideration for all programming interfaces is safety. It is often desirable to have the user wear the hearing aid while it is being programmed, so that the "fit" between the new program and the user's hearing deficiency can be immediately checked. If the user is wearing the hearing aid while it is being programmed, there must be electrical isolation between the hearing aid wearer and the programming system, especially if the programming system is connected to line voltage (120 volts or higher). Many systems use isolated power supplies or battery power and supply all signals to the hearing aid wearer through optoisolators. Wireless systems overcome the problems if isolation from line voltage, but may require optoisolators even if a battery powered, body-worn programming interface is used.